The present invention is in the field of nuclear magnetic resonance spectroscopy instrumentation and relates particularly to the rf probe structure of such instruments.
The practice of radio frequency resonance spectroscopy can be regarded as inclusion in an rf circuit of a class of reactive components which are distinguished physically by quantum mechanical characteristics. Manipulating and detecting quantum transitions of magnetic moments of nuclei, electrons, atoms and molecules has proven to be an abundant source of powerful analytic techniques.
The coupling of excitation and detection portions of the apparatus to the sample is accomplished in the probe. The basic apparatus of the probe comprises an inductive coupling means for transferring energy to or from the sample quantum system under study. A variety of functions are accomplished in the probe apart from excitation of the resonance conditions and detection thereof. For example, the decoupling of interacting magnetic moments of gyromagnetically resonant structures and substructures is often accomplished by irradiating the system with appropriately tuned saturing rf radiation. Furthermore, the separate excitation and detection of a precisely known gyromagnetic resonance is often undertaken concurrently or in a time sharing mode in order to establish a field frequency relationship for instrument stabilization. In various probes of the prior art, several of these functions have been accommodated in common inductive structures with appropriate multiplexing. In other prior art probes, specialization of function has resulted in a multiplicity of substructures and circuits within the probe. Moreover, probe structures often contain additional mechanical apparatus for spinning the sample at high rotational frequencies within the magnetic field. This subject is outside the scope of the present invention.
The present invention is directed to a probe structure which includes an observe coil for exciting and detecting resonance and a decoupler coil for destroying the interaction of the magnetic moments of selected interacting quantum systems. These coils are perforce in close physical proximity with the result that a small portion of the resonant rf signal induced in the observe coil is coupled into tuned circuit of the decoupler coil where this small portion of the rf signal is then dissipated. The loss of this portion of the signal degrades the signal-to-noise ratio at the input of the observe signal preamplifier.
Although the decoupler coil is nominally passive during the observe phase of the experiment, it remains present as a passive tuned finite impedance inductively coupled to the observe coil.
It is an object of the present invention to achieve an enhanced signal-to-noise ratio in the observe channel of a two-coil rf probe of an NMR spectrometer.
In one feature of the invention, the decoupler coil is connected in series with a pair of rf diodes, the latter connected back-to-back.
In another feature of the present invention, the rf diodes are selected such that the small signal derived from coupling a portion of the observe coil signal into the decoupler coil is insufficient to turn on the diode.